In the production of vinyl acetate homopolymers and copolymers by radically initiated emulsion polymerization in water, polymer formed in the process is deposited as scale on the inner walls and on the internals of the polymerization reactor. These deposits hinder the polymerization, since the scale reduces heat exchange via the reactor wall, and the cooling of the reactor becomes more difficult. Additionally, small pieces of the wall scale may become detached and contaminate the polymerization product. In order to eliminate these disadvantages, the polymerization reactor would have to be cleaned to remove the deposits after each production cycle. That, however, would result in a significant extension to the polymerization times and would therefore worsen the economics of the polymerization process.
Disclosed in the prior art is the coating of the inner walls of the polymerization reactor, and also the internals in the polymerization reactor, such as stirrers or cooling facilities or metering devices, with a scale inhibitor (antifouling agent).
In EP 0 052 421 A1, for the purpose of preventing deposits during the polymerization of vinyl chloride, the recommendation is given to use scale inhibitors comprising condensation products of 1-naphthol and formaldehyde in alkaline, aqueous solution or in organic solution. To improve the adhesion of this inhibitor to the reactor wall, the addition of protective colloids, such as of partially hydrolyzed polyvinyl alcohol, for example, is recommended for dissolving the scale inhibitor. A disadvantage of these condensation products is their dark coloration.
To lighten the coloration, WO 97/08210 A1 recommends condensation products of hydroxy-substituted naphthalene compounds and hydroxymethanesulfinic or hydroxymethanesulfonic salts as scale inhibitors. These inhibitors are sprayed with steam on to the reactor walls, the adhesion being said to improve in line with the temperature of the steam.
In order to avoid the dark coloration of the scale inhibitor, WO 96/35723 A1 recommends a condensation product which is obtained by condensing a mixture of a 1-naphthol and a sulfoxylate, preferably a sodium hydroxymethanesulfinate.
WO 96/35724 A1 describes how, in order to improve the adhesion of the scale inhibitor specified in WO 96/35723 A1, it is to be applied by spraying with a steam spray at maximum temperature.
The recommendation in WO 2013/053895 A1, for improving the adhesion of the condensation products used as scale inhibitors, is to spray them on to the reactor wall in a mixture with an acrylic ester polymer.
For the purpose of suppressing the formation of scale during polymerization processes involving vinyl chloride or vinyl acetate, WO 98/24820 A1 recommends condensation products of an aldehyde, a phenol compound, and a hydroxy-substituted aromatic carboxylic acid in the form of their aqueous solution. WO 03/085003 A1 recommends improving the adhesion of these condensation products to the inner wall of the polymerization reactor by setting the pH of the solution at a pH of less than 5. For this purpose, in practice, the aqueous solution of the condensation product is mixed in line with an aqueous acid solution in a pipe conduit leading to the polymerization reactor, and the mixture in the reactor is sprayed on to the walls. A disadvantage is that in this case there may be premature agglomeration in the pipe conduit.